The specific star formation rate of high redshift galaxies: the case for two modes of star formation - Astrophysics > Cosmology and Nongalactic Astrophysics

The specific star formation rate of high redshift galaxies: the case for two modes of star formation - Astrophysics > Cosmology and Nongalactic Astrophysics - Descarga este documento en PDF. Documentación en PDF para descargar gratis. Disponible también para leer online.

Abstract: We study the specific star formation rate SSFR and its evolution at$z\gtsim 4$, in models of galaxy formation, where the star formation is drivenby cold accretion flows. We show that constant star formation and feedbackefficiencies cannot reproduce the observed trend of SSFR with stellar mass andits observed lack of evolution at $z>4$. Model galaxies with $\logM * \ltsim9.5$ M${\odot}$ show systematically lower specific star formation rates byorders of magnitudes, while massive galaxies with M$* \gtsim 5 \times 10^{10}$M${\odot}$ have up to an order of magnitude larger SSFRs, compared to recentobservations by Stark et al

To recover these observations we apply anempirical star formation efficiency in galaxies that scales with the host halovelocity dispersion as $\propto 1-\sigma^3$ during galaxy mergers. We find thatthis modification needs to be of stochastic nature to reproduce theobservations, i.e. only applied during mergers and not during accretion drivenstar formation phases. Our choice of star formation efficiency during mergersallows us to capture both, the boost in star formation at low masses and thequenching at high masses, and at the same time produce a constant SSFR-stellarmass relation at $z\gtsim 4$ under the assumption that most of the observedgalaxies are in a merger triggered star formation phase. Our results suggestthat observed high-z low mass galaxies with high SSFRs are likely to befrequently interacting systems, which experienced bursts in their starformation rate and efficiency mode 1, in contrast to low redshift $z \ltsim3$ galaxies which are cold accretion-regulated star forming systems with lowerstar formation efficiencies mode 2.